In the copending application there is taught a socket testing apparatus, such as a burn-in test socket, for integrated circuit "chips". While the invention hereof is not limited to the use of a burn-in test socket, it nevertheless represents an ideal situation where precise alignment of the test socket to a planar electronic device, such as a motherboard, is required.
In the case of a burn-in test socket, to precipitate early chip failure, if there is going to be a failure, the chip is "exercised" or powered while being subjected to relatively high external temperatures. Typically, a batch of chips is electrically powered in an oven where the temperature is maintained at approximately 150.degree. C. for an extended period of time, such as 1,000 hours. This is referred to in the art as "burn-in".
During burn-in, a batch of chips may be mounted on a mother board, and the chip leads are electrically connected to respective circuit elements on the mother board by a suitable means, such as one or more flexible electrical connectors. Maintaining good and reliable electrical contact between the chip leads and the flexible electrical connector is very important. The copending application represents one way of maintaining a proper force between the chip socket and the mother board by the use of a spring-loaded floating cover member. However, one of the difficulties encountered in mating the chip leads to the circuit elements or pads of the motherboard lies in the alignment of the respective components, particularly with fine, highly dense circuit and pad spacings. That is, the task of aligning the device leads with the board pads via a socket becomes increasingly difficult as the centerline distance and lead size become smaller. Typically the alignments of the device-to-socket and the socket-to-board are done by a hole/pin arrangement. The mechanical size and position tolerance build-up will be the limiting factors in such an assembly.
The present invention avoids the difficulties or problems of the prior methods by a system which takes advantage of the extremely precise photo etching process which is used for the manufacturing of the traces and pads on a printed circuit board. As will be apparent in the description which follows, dimensioning and manufacturing of the socket and board are done from two perpendicular lines or planes which are dedicated as origins or datum lines. It is now understood that ideal alignment between the two devices will occur when the datum lines from the socket overlap those from the board within the allowable tolerance range. This concept completely eliminates the need for the use of a hole/pin alignment between the socket and the board. The unique system of this invention is equivalent to a tolerance reduction of about 75%. This will become clearer in the following specification, particularly when read in conjunction with the accompanying drawings.